The present invention relates generally to the field of laser surgery. More specifically, the present invention relates to a laser surgical procedure for treating glaucoma.
Glaucoma affects more than two million Americans and is the leading preventable cause of blindness in the United States. Glaucoma results from the body's inability to drain the clear, transparent liquid called the "aqueous humor", which flows through the inner eye continuously. Most commonly, the aqueous fluid drains from the anterior chamber to the sclera, through a variety of drainage channels or canals, such as the trabecular meshwork, the ciliary body, and through a natural channel in the eye called Schlemm's canal. These channels can become smaller with age, as they are clogged by deposits which build up slowly over time. In other cases, the channels are misformed at birth, and optimal drainage cannot be achieved without surgical intervention.
Without proper drainage of the aqueous humor from the anterior chamber, an abnormally high fluid pressure results within the eye which is referred to as glaucoma. As pressure builds up, the pressure can "pinch" both the optic nerve and the blood vessels which nourish the retina. The result is usually a slow loss of peripheral vision, and eventually blindness.
To treat glaucoma, it is conventional to form a channel in the sclera of the eye to drain aqueous fluid from the anterior chamber of the eye, thereby reducing the fluid pressure. The exit opening of the channel is covered by conjunctival tissue, which provides a filtering bleb. Typically, the channel in the sclera is made by a knife or other mechanical devices. These mechanical devices cause great trauma to the scleral tissue. Such trauma results in the formation of scar tissue which eventually obstructs the channel. Once the passageway is occluded, the aqueous humor will begin to build up in the anterior chamber again, which will result in the return of excess intraocular pressure and the failure of the procedure.
While it is possible that formation of a larger opening in the sclera would take longer to fill with scar tissue, if the hole is too large, an excess of aqueous humor will drain from the anterior chamber and will result in hypotony (i.e., excessively low intraocular pressure). Accordingly, when performing a standard glaucoma filtering procedure, it is desirable to ensure that the scleral opening is of sufficient size to allow for normalization of intraocular pressure, but not large enough to produce hypotony. If the opening is too small, the opening will close due to the build-up of scar tissue, and the procedure fails. Variables, including the degree of operative trauma caused by the mechanical procedure and the individual patient's response to this trauma, make it difficult to reliably predict the length of time a channel formed by a mechanical device will remain open. Typically, such channels remain open for at least one year. Since glaucoma tends to affect the elderly, it is not desirable to have to reschedule surgery on the eye on a yearly basis.
Recently, lasers, such as Nd:YAG and thalmium:YAG lasers, have been used as an alternative to mechanical devices to form the channels in the sclera tissue. These lasers, while providing less mechanical trauma to the sclera tissue than the mechanical procedures, still produce a high amount of heat and cause thermal trauma to the sclera. Since any type of trauma triggers the body's natural healing response, channels formed by Nd:YAG or thalmium:YAG lasers will eventually begin to form scar tissue that occlude the channel. Channels in the sclera formed by Nd:YAG or thalmium:YAG lasers remain open for 4-5 years at best.
Another common glaucoma treatment procedure involves forming a channel in the sclera to enable drainage of the aqueous humor using either a laser or a mechanical device and placing a stent in the channel in an attempt to keep the channel from narrowing. The stent may be effective to inhibit closure; however, the body may detect the stent as a foreign object and may start to form scar tissue at the stent site. Eventually the scar tissue can grow over the opening on either side of the stent and prevent drainage of the aqueous humor through the channel. Once the channel is closed, the stent must be removed and replaced, which involves additional surgical procedures.